This proposal continues ongoing studies related to alterations in vitamin D and parathyroid hormone (PTH) in chronic renal failure (CRF). Specifically, two areas will be investigated: 1) ALTERATIONS IN VITAMIN D RECEPTOR EXPRESSION AND ACTION 2) EXPRESSION AND REGULATION OF RENAL AND EXTRA-RENAL 1-HYDROXYLASE. Vitamin D receptor (VDR) levels are decreased in parathyroid glands of patients and animals with CRF. The decrease serum 1,25(OH)2D3 (a stimulator of VDR levels) and increase PTH (a suppressor of VDR) in CRF are likely causes of the low VDR levels. The proposed studies will examine the roles of 1,25-(OH)2D3 and PTH in the regulation of VDR in the parathyroid glands and intestine of rats with CRF. VDR will be analyzed for 1,25-(OH)2D3 and DNA binding affinity, and quantitated by 1,25- (OH)2D3 binding and by Western and Northern blot analysis. Cultured parathyroid cells will be used to examine the direct effects of 1,25- (OH)2D3, PTH, and other factors on the levels of VDR and its DNA binding affinity. In addition, studies will be performed in uremic patients to determine if the concentration of circulating 1,25-(OH)2D3 per se has a role in the suppression of PTH. We will also determine if the administration of 1,25-(OH)2D3 to patients newly starting dialysis modifies the natural history of secondary hyperparathyroidism. Various degrees of vitamin D deficiency are associated with CRF. Despite high PTH, a stimulator of renal 1alpha-hdroxylase, the remnant enzyme is unable to compensate 1,25-(OH)2D3 production. However the very low levels of 1,25-(OH)2D3 in anephric patients can rise dramatically following 25(OH)D3 treatment suggesting a role for substrate availability in extrarenal 1.25-(OH)2D3 generation. The affinity for 25(OH)D of the extrarenal 1alpha-hydroxylases of liver and peripheral macrophages is reduced in uremia. We will examine the associated between the expression of extrarenal 1-alpha-hydroxylase and the reduction in serum 1,25-(OH)2D3 with the progression of renal failure. Since the regulation of extrarenal 1 alpha-hydroxylase differs markedly from that of the renal enzyme, we will examine the modulation of extrarenal 1,25-(OH)2D3 production as a potential therapeutic tool to prevent the alterations in calcium homeostasis association with calcitriol deficiency in severe renal failure.